1. Field of the Invention
The present invention relates to a display apparatus, and more particularly, to a display apparatus in which data lines of a display panel is driven in a time divisional manner.
2. Description of Related Art
Typically, output amplifiers are integrated in a data driver IC for driving data lines in a liquid crystal display panel and other display panels. This is because load of the data line such as parasitic capacitance, wiring resistance and on-resistance of TFT is large. The output amplifier is necessary to quickly drive the data line having the large load to a desirable voltage.
One problem lies in the point that when the number of data lines is increased, the number of output amplifiers is also required to be increased. In the display panel in recent years, the number of pixels is increased more and more. Thus, the number of data lines is also increased, so that the number of output amplifiers provided to drive the data lines tends to be increased. However, the increase in the number of output amplifiers causes the following problems. The first problem lies in the increase in the chip area of the data driver IC when the number of output amplifiers is increased. The increase in the chip area of the data driver IC is not preferable because this involves the increase in cost of the data driver IC. The second problem lies in the increase in the steady-state consumed power of the data driver IC. Since a steady-state current flows through the output amplifier according to a power supply voltage, the output amplifier consumes a certain power in a steady-state state. Thus, the increase in the number of output amplifiers causes the increase in the consumed power as the entire data driver IC, and this is not especially preferable in case that a display apparatus is used in a field which requests the small consumed power such as a mobile terminal.
One measure to cope with this problem is to employ a time divisional driving method. The time divisional driving method is a technique that sequentially selects the data line to be driven with the output amplifier by a demultiplexer. In the time divisional driving method, one output amplifier is used to drive data lines. Thus, the number of output amplifiers integrated in the data driver can be reduced.
A hardware configuration for attaining the time divisional driving method is mainly divided into two kinds. In one kind of hardware configuration, demultiplexers (switch) are integrated in the display panel to select the data line, as disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-327518) and Japanese Laid Open Patent Application (JP-P2005-43418A). In the other kind of hardware configuration, switches are integrated in the data driver IC to select the data line, as disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 5-173506), and Japanese Laid Open Patent Applications (JP-P2002-318566A and JP-P2006-154808A).
FIG. 1 is a conceptual diagram showing the configuration of a liquid crystal display apparatus in which a demultiplexer is integrated in a display panel to select data lines. In FIG. 1, a liquid crystal display apparatus 100 contains a liquid crystal display panel 101. Scanning lines G, data lines D and pixels 103 are integrated in an effective display region 102 of the liquid crystal display panel 101, i.e., a region that is actually used to display an image in the liquid crystal display panel 101. The scanning lines G extend in an x-axis direction, and the data lines D extend in a y-axis direction. The pixels 103 are provided at intersections of the scanning lines G and the data lines D.
A circuit group for driving the pixels 103 is provided around an effective display region 102. Specifically, a scanning line driver circuit 104 and a demultiplexer 105 are integrated in the liquid crystal display panel 101. Moreover, a data driver IC 106 is connected in a flip-flop manner to the liquid crystal display panel 101. Attention should be paid to the description of the liquid crystal display apparatus 100 in FIG. 1, in which a COG (Chip on Glass) technique is employed to mount the data driver IC 106. The demultiplexer 105 is configured by switches 105a provided between the data lines D and output nodes of the data driver IC 106. The liquid crystal display apparatus 100 in FIG. 1 is configured in such a manner that the 6 data lines D are selectively connected to the output node of one data driver IC 106. When the pixel 103 is driven, the 6 data lines D are sequentially selected by the demultiplexer 105, and a drive voltage is supplied from the output node of the data driver IC 106 through the selected data line D to the desirable pixel 103.
The chip width of the data driver IC 106 is smaller than the width of the effective display region 102. Thus, wirings 107 to connect the output node of the data driver IC 106 and the demultiplexer 105 are radially arranged. The region in which this wirings 107 are arranged is referred to as a throttling region 108. The existence of the throttling region 108 is not preferable because of the increase in the region that is not used to actually display the image in the liquid crystal display panel 101.
On the other hand, FIGS. 2 and 3 are conceptual diagrams showing the configuration in which the demultiplexer is integrated in the data driver IC to select the data line. In a liquid crystal display apparatus 100A of FIG. 2, the demultiplexer is integrated in a data driver IC 106A and not in a liquid crystal display panel 101A. The data line D is directly connected to the output node of the data driver IC 106A through the wiring 107 that is laid in the throttling region 108.
FIG. 3 is a block diagram showing a typical configuration of the output stage of the data driver IC 106A. The image data, i.e., a pixel data to specify the gradation of each pixel is sent to a digital-to-analog (D/A) converter (DAC) 111, and the D/A converter 111 supplies a gradation voltage corresponding to the pixel data to an output amplifier 112. The output of the output amplifier 112 is connected to a demultiplexer 113. The demultiplexer 113 sequentially selects data lines D and connects the selected data line D to the output of the output amplifier 112. A drive voltage is supplied from the output node of the data driver IC 106A through the selected data line D to the desirable pixel 103.
Japanese Laid Open Patent Application (JP-P2005-165102A) further discloses the improvement of the configuration in which a demultiplexer to select the data line is integrated in the data driver IC. In the data driver IC disclosed in this related art, the demultiplexer is integrated in the data driver IC to connect the output amplifiers to output nodes, and a signal line to connect the output node, which is not connected to the output amplifier, to the output of a D/A converter is provided.
One demand to the display apparatus in recent years is to increase the number of data lines that can be driven by one data driver IC. In order to cope with this demand, the number of data lines that are driven in a time divisional manner by one output amplifier is required to be increased. Specifically, in the liquid crystal display apparatus of a next generation, it is required to use one output amplifier and drive the six or more data lines.
Another demand is to reduce a region other than an effective display region in the display panel (hereinafter, a non-effective display region). Through reduction of the non-effective display region it is possible to reduce the size of the display apparatus when the display panel is mounted, and this is useful for decreasing cost of the display panel.
However, the above two kinds of hardware configuration have a problem that, when the number of data lines to be driven in a time divisional manner by one output amplifier is increased in association with the increase in the number of data lines to be driven by one data driver IC, the non-effective display region of the display panel is increased.
At first, in the configuration in which the demultiplexer is integrated in the display panel, the increase in the number of data lines to be driven in the time divisional manner by one output amplifier involves the increase in the area of the demultiplexer 105. This results in the increase in the area of the non-effective display region in the display panel. There are two reasons why the non-effective display region is increased. Firstly, the trial of the increase in the number of data lines to be driven in the time divisional manner by the output amplifier requires the increase in the gate width of TFT of the demultiplexer provided on the display panel. The increase in the number of data lines to be driven in the time divisional manner by the output amplifier decreases a drive period of one data line. In order to sufficiently drive the data line in a shorter drive period, the on-resistance of the TFT of the demultiplexer is required to be low. In order to decrease the on-resistance of the TFT, the gate width of the TFT must be increased. However, the increase in the gate width of the TFT of the demultiplexer leads to the increase in the non-effective display region. Secondly, the increase in data lines to be driven in the time divisional manner by the output amplifier requires the increase in the number of control signal lines that are used to send control signals to the switches. This increases the area of the non-effective display region. The control signal line to send the control signal to the switch is a long wiring that reaches from one end of the effective display region of the display panel to the other end, and the area occupied thereby is very large.
On the other hand, in the configuration in which the demultiplexer for selecting the data line is integrated in the data driver IC, the number of output nodes from the data driver IC is not reduced, and the number of data lines driven by the data driver IC is increased. This increases the height of the throttling region 108 (the dimension in the y-axis direction), and also increases the non-effective display region of the display panel. This reason is as follows. In order to prevent a short-circuit between the wirings 107 to connect the data line D and the output of the data driver IC, a certain interval is required to be reserved between the wirings 107. Thus, an angle θ between the wiring 107 and the line in which the outputs of the data driver are lined up has a predetermined lower limit. Thus, in order to connect the wiring 107 to the data line D of the end, the height of the throttling region 108 is required to be reserved to a certain degree. This leads to the increase in the non-effective display region. Also, in order to suppress the height of the throttling region 108, if the interval between the wirings 107 is narrowed to a degree at which the short-circuit is not generated, a parasitic capacitance between the wirings is increased. Therefore, with the influence of the voltage variation caused by the capacitance coupling, a voltage error becomes greater. In particular, the voltage errors of the pixels located at the left and right ends of the effective display region 102 in which the wiring 107 is long become large, which brings about the display irregularity.